Display devices comprise a plurality of pixels for displaying images, a driver for driving the pixels, etc. The pixels and the driver comprise a thin film transistor as a switching element. Such a thin film transistor is, in some cases, constituted by, for example, a polycrystalline semiconductor layer formed from a polycrystalline silicon or an oxide semiconductor layer formed from an oxide.
In recent years, display devices comprising both a thin film transistor including a polycrystalline semiconductor layer and another thin film transistor including an oxide semiconductor layer have been developed. In the thin film transistor including a polycrystalline semiconductor layer, when a contact hole is formed to connect the polycrystalline semiconductor layer and an electrode to each other, an oxide film is, in some cases, formed on a portion of a surface of the polycrystalline semiconductor layer, which is exposed by the contact hole. Therefore, in order to remove the oxide film, a cleaning process using hydrofluoric acid, for example, is carried out in some cases after forming the contact hole.
On the other hand, oxide semiconductor layers have low resistance to hydrofluoric acid and therefore, when exposed to hydrofluoric acid, they are, in some cases, at least partially vanished. For this reason, in the case where a thin film transistor including a polycrystalline semiconductor layer and a thin film transistor including an oxide semiconductor layer are both formed on the same substrate, the electrical characteristics of the thin film transistor with the oxide semiconductor layer are, in some cases, deteriorated by, for example, a cleaning process using hydrofluoric acid, thereby making it difficult to secure the reliability.